like everyone else said, when they tell you. The first school horse I rode and took lessons on was named dusty, haha I've met adults that learned to ride on him. He was going strong for about 25 years. I find the time you stop riding a horse is the time they die, even if you at least just hop on bareback (or put a small kid on them) it keeps them going and going and going... haha I think they need a perpose, rather then to just stand in the feild, any old horse I've known has died almost right after they had stopped being used for anything.
If there are no horses in heaven... im not going.